This invention relates to a stable, concentrated aqueous fertilizer suspension, which can be diluted with water to provide a working solution in water irrigation systems.
Besides nitrogen, potassium, magnesium and trace elements, calcium and phosphorus are vital plant nutrients, which should be constantly available to plants. Irrigation fertilisation usually comprises water-soluble fertilizers, often prepared as a concentrate solution, added to the irrigation water at intervals. If such a concentrate solution/fertilizer solution contains calcium and/or magnesium salts and ordinary water-soluble phosphorus sources such as for instance monoammonium phosphate, diammonium phosphate or potassium phosphate, the pH of the solution will be so high that calcium forms insoluble phosphate salts. The insoluble component precipitates, thus clogging the irrigation systems, and its nutrients are no longer in a form which the plants can utilise. The trace elements, added as sulphates or nitrates, also tend to precipitate under these conditions, and for this reason expensive chelate forms are normally used to prevent this. Thus the preparation and use of P, NP, PK and NPK fertilizers containing calcium and/or magnesium in irrigation systems are tricky. A solution to the precipitation problems entailed by water irrigation fertilizers containing calcium and/or magnesium and phosphorus could involve supplying the solution containing calcium and/or magnesium and phosphorus by means of two discrete irrigation systems in parallel. However, such an implementation would be complicated and obviously quite costly.
In U.S. Pat. Nos. 5,171,349 and 5,395,418 the phosphorus source consists of water-soluble urea phosphate, which reduces the pH of the fertilizer concentrate solution and helps keeping the calcium in a dissolved state. Here the calcium source consists of calcium nitrate. Nevertheless, this method involves the drawback of an extremely low, 2 to 40% salt content of the fertilizer solution, implying high transport costs, since the fertilizer solution may contain up to 60 to 98% of water. In practical application, the cultivator thus has to prepare the solution himself by blending solid raw materials, taking care that overdosing does not entail unexpected precipitates in the fertilizer concentrate solution. This requires that the cultivator is well informed and has adequate skills, which are not always available for instance in less developed countries. Moreover, a fully water-soluble phosphorus fertilizer raw material is expensive, and may thus be used to a very limited extent in poor countries.
EP patent application 84 196 and U.S. Pat. No. 4,177,052 disclose liquid suspensions containing calcium and phosphorus, but these already contain components that are totally water-insoluble, and due to sedimentation problems, they are unusable in water irrigation systems.
JP patent application 52 75571 discloses a fertilizer solution containing calcium and phosphorus, which has been prepared from hydrated or burnt lime or calcium carbonate by dissolving with a mixture of phosphoric acid, nitric acid and hydrochloric acid, but here too, the low salt content due to the solubility of the fertilizer solution involves a problem. Moreover, this acid mixture would require special anti-corrosive materials in the irrigation system.
The applicant's FI patent application 941759 describes a fertilizer suspension suitable for drip irrigation, wherein the solid particles are of micron particle size.